Glaucoma is a disease of the eye characterized by damage to the optic nerve caused by intraocular pressure which is too high for the nerve to tolerate. Two types of procedures have generally been utilized in the prior art to control glaucoma: (1) decreasing aqueous fluid production and (2) increasing outflow of aqueous fluid from the anterior chamber of the eye. Of the numerous surgical procedures which have been described to control glaucoma, those which result in an improvement of outflow facility are theoretically more advantageous than those designed to decrease aqueous production, since over 95% of glaucomatous disease is a consequence of increased outflow resistance, rather than increased aqueous production or episcleral venous pressure. Operations designed to lower intraocular pressure by decreasing aqueous production have the disadvantage of curtailing aqueous flow to various avascular ocular structures which depend on nutrients supplied by aqueous humor for normal functioning.
The most frequently performed operation for chronic open angle glaucoma in adults is a "filtration" procedure which increases the outflow facility by providing an opening between the anterior chamber and subconjunctival space (between the conjunctiva and sclera) through which aqueous humor can flow to reduce intraocular pressure. The intraocular pressure level following these procedures varies, with an initial overfunction and its associated periods of low intraocular pressure (hypotony). The most common reason for failure of this type of glaucoma eye surgery is due to scarring in the subconjunctival space, thereby restricting the drainage flow from the anterior chamber.
A relatively new type of glaucoma surgery utilizes a valve implant, as disclosed in U.S. Pat. No. 4,037,604 to John Newkirk. The Newkirk device provides a tube which communicates between the anterior chamber of the eye and the subconjunctival space. The implant has a unidirectional valve at the end of a tube which is intended to open at a predetermined intraocular pressure, to release aqueous humor from the anterior chamber of the eye, to thereby reduce intraocular pressure. The open end of the tube is placed into the anterior chamber of the eye, while the valve end of the tube is located in the space between the sclera and the conjunctival tissues.
However, the Newkirk device still suffers failures since the valve implant does not reduce the incidence of scarring of the conjunctival, Tenon's, and/or episcleral tissue to the underlying sclera. Such failures may occur at any time in the post-operative course.
In a series of 79 eyes treated for neovascular glaucoma with the Newkirk apparatus, 53 eyes maintained an intraocular pressure less than or equal to 24 mm Hg with a mean follow-up of two years. Of these 53 "successfully" treated eyes, 10 eyes required bleb (the area in the subconjunctival space to which aqueous humor drains) revision because of scarring in the subconjunctival space with resultant increased intraocular pressure, which occurred between one and eleven months post-operatively. Of the 26 eyes which failed to maintain an intraocular pressure less than or equal to 24 mm Hg, 18 of the failures were secondary to bleb scarring, which occurred from three weeks to twenty months post-operatively. Even after attempted bleb revision in 8 of these 18 eyes, functional filtration could not be restored. Therefore, scarring of the conjunctival bleb resulted in permanent failure, or temporary failure requiring additional surgical intervention, in 28 of 79 eyes. Growth of fibrovascular tissue over the internal portion of the valve implant was responsible for failure in 10 of the eyes in this series.
In an attempt to eliminate bleb scarring, the tube was lengthened and a large Silastic disk was incorporated around the valve, thereby diverting drainage to a more posterior aspect of the eye, and spreading the drainage to a larger area. While this apparatus reduced failure due to bleb scarring, the scarring was not eliminated. Thus, prior art apparatus and procedures were not capable of accurately predicting or setting optimal intraocular pressures in the eye, since it was impossible to predict the amount of scarring and flow resistance in the bleb wall.
While many different forms of medical therapy have been used in an attempt to prevent the scarring of conjunctival, Tenon's and/or episcleral tissue over a drainage site, none have been shown to achieve 100% success, and all are associated with adverse effects.